This invention relates generally to alignment systems for achieving precise relative alignments of a first object, such as a photomask or a reticle, and a second object, such as a semiconductive wafer or a photomask, and, more particularly, to apparatus for use in such systems to facilitate precise positioning of the first object with respect to an axis of motion of a movable stage for holding the second object.
In the semiconductor industry step-and-repeat projection alignment and exposure systems are employed both in the fabrication of photomasks and in the processing of semiconductive wafers to form integrated circuits and the like. A high (sub-micron) resolution photomask is typically fabricated by utilizing a precisely controlled stage movable along coordinate axes of motion to successively position adjacent regions of the photomask with respect to an image (formed by a projection lens) of a reticle containing a level of microcircuitry that is printed on the photomask at each of these regions. This step-and-repeat printing operation forms an array of adjacent regions of microcircuitry of one level on the photomask in rows and columns parallel to the coordinate axes of motion of the stage. A set of such photomasks, each bearing an array of microcircuitry of a different level, is typically employed in the fabrication of integrated circuits or the like from a semiconductive wafer. In the course of this fabrication, the semiconductive wafer is sequentially aligned with each photomask of the set, and the level of microcircuitry printed on the photomask is in turn printed on the semiconductive wafer (this could also be performed in a step-and-repeat printing operation by utilizing a precisely controlled stage movable along coordinate axes of motion to successively position adjacent regions of the semiconductive wafer with respect to each of the photomasks). It is also possible to eliminate the operation of fabricating a set of such photomasks by employing a precisely controlled stage movable along coordinate axes of motion to successively position adjacent regions of the semiconductive wafer with respect to each of the reticles employed in fabricating the set of photomasks so that the level of microcircuitry contained on each of those reticles may be printed directly on the semiconductive wafer at each of those regions during separate step-and-repeat printing operations.
In order, for example, to facilitate the precise positioning or alignment of one level of microcircuitry being printed on a semiconductive wafer at each of an array of adjacent regions thereof relative to another level of microcircuitry previously printed or yet to be printed on the semiconductive wafer at each of those same regions, it would be highly desireable to employ a precisely controlled stage having a visible indicuim of at least one of the coordinate axes of motion of the stage so as to facilitate the precise and repeatable positioning of a photomask or a reticle with respect to that axis of motion of the stage. Unfortunately, however, the stages employed in conventional step-and-repeat alignment and exposure systems do not have such an indicium.
Accordingly, it is the principal object of this invention to provide an improved alignment system with a precisely controlled movable stage having a visible reference mark that is indicative of at least one of coordinate axes of motion of the stage.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished according to the illustrated preferred embodiment of this invention by providing a step-and-repeat alignment and exposure system including a main stage controlled for movement to different positions along orthogonal X and Y axes; a chuck mounted on the main stage for supporting a semiconductive wafer thereon; a substage mounted on the main stage for aligning a reference mark on the substage with one of the X and Y axes of motion of the main stage; another stage controlled for aligning an image of a reticle supported thereon with the reference mark; a projection lens mounted between the main stage and the other stage for imaging illuminated portions of the reticle onto portions of the reference mark or the semiconductive wafer, depending on the position to which the main stage is moved; a light source for directing illumination and exposure light along an optical path extending through the reticle; a filter mounted for passing light of a wavelength appropriate for the projection lens along that optical path to the reticle; a shutter mounted for selectively controlling the passage of light along that optical path to the reticle; a mask plate mounted for controlling the portions of the reticle illuminated by the light passing along that optical path when the shutter is opened; a beam splitter mounted between the projection lens and the reticle for providing a viewing port at which an aerial image of the portions of the reference mark or semiconductive wafer illuminated by the projected image of the illuminated portions of the reticle may be viewed; and an objective lens unit for use with an ocular lens unit to permit viewing of the aerial image provided at the viewing port while the main stage is controlled to directly align either an image of the reticle with the reference mark or the semiconductive wafer with the reticle.